1. Field of the Invention
This invention relates to a metal plated material having an anticorrosive film and to an aqueous solution for forming the same, and more particularly, to a metal plated material used for a conducting portion of an eletronic part, such as a connector, and an aqueous solution for forming the same.
2. Description of the Prior Art
Various kinds of connectors are used for making connections in an electronic device. The electronic parts, such as connectors, have contacts, and those contacts of which a particularly high level of reliability is required are formed by gold plating owing to the good conductivity and inertness of gold.
Gold is, however, expensive and adds to the cost of making a part or device. Therefore, the gold film is usually formed with a small thickness to enable a reduction of the cost. In accordance with decreasing a thickness, the film has a large number of pinholes. The pinholes are likely to cause the corrosion of the base metal which raises the contact resistance and lowers its reliability.
Various methods have, therefore, been taken to prevent corrosion through pinholes. They include the steps of forming a chromic acid film, dipping or immersing in a solution prepared by dissolving an organic compound having an adsorbing power in an organic solvent containing a halogen, and applying a lubricant oil to cover pinholes.
The chromic acid film has, however, been unsatisfactory in its action for preventing corrosion, since it is a film of an inorganic substance which is easily decomposed in a corrosive environment containing an acid gas. An increase of film thickness has been necessary to achieve an improved result, but has brought about a higher contact resistance and a lower reliability, as well as a problem of environmental pollution.
The problem of environmental pollution associated with solvent use is another disadvantage of prior art methods that employ an organic compound in an organic solvent for imparting anticorrosive properties.
On the other hand, the method has been carried out of applying a contact oil which consists of a lubricant oil by adding with an organic compound for preventing corrosion. It has been effective in a moist environment, but of low effect in a corrosive environment containing an acid gas. A number of reasons exist for which corrosion easily occurs. The compound is not effectively adsorbed to the surface to be protected. The molecules of corrosive gases and water are so small as to pass through an oil layer and reach the surface to be protected. Dust easily adheres to the oil layer and causes the gas and water molecules to be adsorbed to the surface to be protected.
Therefore, it has been necessary to find a method which can prevent corrosion as effectively as, or more effectively than the prior methods as described above without causing any environmental pollution, and without bringing about any appreciable increase in contact resistance. In this connection, there has been proposed a method which uses an aqueous solution of an organic compound having an anticorrosive action for application by dipping or electrolysis (Japanese Patent No. 2,717,062; hereinafter referred to as Prior Art 1).
According to the disclosure of Prior Art 1, an aqueous solution consisting mainly of a mercaptobenzothiazole derivative is electrolyzed by a direct current, while a gold plated material serves as the anode. The gold layer is formed on Nickel. The Nickel is electrolytically oxidized under the pinholes in the gold layer, and its reaction with the mercaptobenzothiazole derivative forms a complex compound which fills and seals the pinholes. The sealing of the pinholes is, however, insufficient for preventing any contact failure.